The invention generally relates to an arrangement and method for removal of particulates in a gas flow. In particular, the invention relates to an arrangement and method for removal of particulates in a gas flow applied to purify exhaust gas from an internal combustion engine, preferably a diesel engine in a mobile application.
The need for reducing the amount of particulates (ash, soot, organic fractions etc.) in exhaust gas emanating from diesel engines has become increasingly important.
Generally, prior art particulate filters makes use of a ceramic monolithic structure with porous walls in which adjacent channels are plugged either at the inlet side or at the outlet side. Thereby the gas is forced to flow through the walls in which the particulates get stuck. A problem with this type of filter is clogging of the walls leading to increasing pressure drop which makes it necessary to frequently regenerate the filter. In addition, clogging of the walls increases the risk of uncontrolled regeneration that may destroy the filter. Further, service stops are normally required for removing ash from the filter.
Another principle is to use an open, non-plugged, structure made of a metallic non-porous material where the channel walls are adapted to give rise to turbulence in the gas flow so that particulates come into contact with, and are deposited onto, the channel walls. A problem with this type appears to be a low “filtration” efficiency, i.e. only a small fraction of the particulates sticks to the walls, which makes the device large if high efficiency is required.
It is desirable to provide an arrangement for removal of particulates in a gas flow that separates particulates in an efficient way and that at the same time has a potential of being very resistant towards clogging. It is also desirable to provide a method for removal of particulates in a gas flow that combines separation efficiency with a potential of being clogging resistant.
An aspect of the invention concerns an arrangement for removal of particulates in a gas flow, comprising at least one gas passage that is at least partly surrounded by a porous material, and it is characterized in that the arrangement comprises means for exposing the gas passage to sound that induce gas movements in a direction towards the porous material. An advantageous effect of this feature is that particulates present in the gas flow will, at least partly, follow the movements of the gas resulting in that the movements of the particulates can be controlled as to force the particulates to be trapped onto, or inside, the porous material without having to block the main gas flow by forcing it through a porous material. This makes it for instance possible to use open, i.e. non-plugged, gas channels separated by porous walls and still get a large fraction of the particulates stuck on the walls. Such a design is efficient in separation and at the same time much more resistant towards clogging and increased pressure drop compared to prior art filters using plugged channels, hi addition to direct movement of particulates, the sound will create turbulence in the flow which further increases the deposition of particulates onto the porous material. The inventive arrangement is useful also for plugged filters and other types of filtering devices since it can be used to, for instance, direct particulates to certain parts of the porous material or filter, or to agglomerate particulates to form larger particles.
In a first aspect of the invention the sound exposure means is arranged to direct the sound in a direction that differs from a main direction of the gas flow. Preferably, the sound exposure means is arranged to direct the sound at an angle of at least 30°, preferably at least 60°, preferably substantially perpendicular, in relation to the main direction of the gas flow, in most applications the advantageous effects of exposing the gas passage to sound will increase with said angle up to a maximum at a point where the sound is perpendicular to the main direction of the gas flow.
An aspect of the invention also concerns a method for removal of particulates in a gas flow, comprising the step of leading the gas flow through at least one gas passage that is at least partly surrounded by a porous material. The inventive method is characterized in that it comprises the step of exposing the gas passage to sound that induce gas movements in a direction towards the porous material.